DX50 and Pico Power
The Quickstep has Jan Meier's active balanced ground and it is as fast as any portable amp that I have heard. Attack and decay are excellent. It also has a sound signature which is quite unique. I have put much thought into how to describe it and what I have come up with is "cool lush". It is not a warm amp but it is not strictly 'gain by wire' either. If a warm amp can be describes as akin to walking into the steamy foliage of a jungle, the QS is like walking through a borealis forest on a cool summer night. If that imagery doesn't work for you, tough, it's my post. What this means is that the sound of the QS is actually quite full but it is not adding warmth to the already slightly warm DX50 line out. With reasonably neutral-of-tone IEMs like my ES5 and Flat-4 Kaede, the results are stunning. I would describe the sound as luxurious. Yes, an amp can actually sound luxurious; legions of Meier fans know something that you don't. The QS does everything right combined with the DX50, not a harsh note in the trebles, lovely mids and solid fast bass. The one negative that I will throw out there is the combination of DX50 + QS with a colored headphone like my euphonic Final Audio Heaven VI does not produce absolutely ideal synergy. The QS + Heaven VI with a neutral DAC, and the DX50 + Heaven VI with a less lush amp can both be excellent with the right third component. The three together though, while certainly sounding very good, is nonetheless a little "rich" of sound to my tastes.
DX50 and Pico Power
If the QS sound can be described as cool lush, I would say the the operative words for the Pico Power are "boldly honest". Well, we have to be careful here. The PP has a discrete output stage using NOS transistors rather than the standard output buffers. The result is a sound that is a bit relaxed in the upper register. There is a nice organic quality to the treble. Still I use the terms "boldly honest" for the PP because it just throws so much dang power into the mix without a hint of the QS lushness. The result is a great sense of presence. The QS does not lack for transparency but the PP ups the ante. This synergizes perfectly with the thin but slightly warm line out of the DX50, even with colored IEMs like the Heaven VI. To my ear, if the QS is the more luxurious sounding of the two amps, the PP is the more honest. It has a huge sound that just sounds true to source. The other quality that distinguishes the Pico Power is the bass. The QS is no slouch in this department but for driving IEMs, the PP has the best bass that I have heard in a portable amp. It has remarkable power and impact without a hint of bloat. The DX50 line out is frankly not a bass monster and the PP's ability to resolve so much bass detail and generate so much impact with the DX50 is extremely welcome. (Note also that this is not the product of any artificial bass boost; the Inner Fidelity FR tests show that the PP is utterly neutral.)
Thanks for that info and impressions; you`re pushing me towards the PP. I`m still trying to imagine using 9 volt batteries, and I like the idea of always being able to buy standard disposable batteries in a pinch. (It`s one of the benefits of using a dSLR with a battery grip)
BTW, you labeled both paragraphs as Pico Power, and I think a lot of people would like to read that mini-review so you might want to edit it.....